I do not know, we are doing hundreds of tests with different bootloader, each micro work (or not) in a different way =(But if you program via ISP, the serial works, however, if the bootloader loads, it is very difficult to upload a sketch via serial In some cases just a R, in others a RC, in others nothing works

Me too.Every people has tried to program this chip has encountered problems solved with different solutions.I think the reason can be related to the use of breadboard/protoboard... but I can not replicate those test on a PCB because I don't make PCBs at home